1. Field of the Invention
This invention relates to sandwich panels which are used where strong, stiff, lightweight panels are required, and more particularly to exceedingly lightweight sandwich panels readily formed to different and varying characteristics of rigidity, strength, compressibility, flexibility, profile, shape, size, thickness, weight, etc., and methods of so doing.
2. Description of the Prior Information
Sandwich panels are not new. Basically, they consist of two outer layers interconnected by a core layer. The outer layers may be sheets or slabs of various material such as wood, metals, paper, or plastics. The inner or core layer usually is particle board, plywood, woodstrips, plastic sheeting or a honeycomb structure (see U.S. Pat. Nos. 3,533,894 (Engelbrecht et al), 3,462,330 (Grieg et al); 4,643,933 (Picken) and 4,731,038 (Hancock et al)) of paper or aluminum or plastic to provide lightness while separating the outer layers a distance sufficient to provide the necessary resisting moments for the uses contemplated; the characteristics of a honeycomb panel being controlled not only by core material thickness, but also by density and cell design. The edges of honeycomb or other core structure are secured to the outer layers usually by chemical bonding. Honeycomb edges are narrow and hence the bonding is weak, can be easily pulled apart, and results in many sandwich panel failures. Furthermore, it does not allow for much flexibility in altering the properties of a sandwich panel.
Sandwich panels have also been formed of inner cores involving pulled apart plastics. Such cores are formed by placing a plastic sheet between heated molds of which one is vented on the underside to atmosphere. After the mold is closed and the plastic warmed, the molds are separated a distance equal to the thickness of the finished core. A core having a definetly random cellular structure results.